This invention relates to a liquid crystal display, particularly, having improved temperature dependence of threshold voltage, stably high contrast in a broader temperature range and an improved switching time at room temperature and low temperature ranges.
Not being limited to indoor use, liquid crystal displays (LCDs) have been extending their outdoor use under severe temperature conditions, such as in personal digital assistances (PDAs) for computers, automotive instruments, and other instruments for outdoor use. With this broadening of application, reduction in display contrast of LCDs due to surrounding temperature change has given rise to a problem. The reduction in LCD display quality with surrounding temperature change is largely attributed to the temperature dependence of the threshold voltage Vth. Conceivable causes of the Vth change with temperature include changes of elastic coefficients dielectric constant, etc. of nematic liquid crystals with temperature and changes of intrinsic pitch of a chiral dopant added with temperature. It has been proposed to improve temperature dependence of a threshold voltage by controlling change with temperature in intrinsic pitch of a chiral dopant (see JP-A-55-38869). However, this technique is disadvantageous in that the effect varies depending on the combination of mother liquid crystals and a chiral dopant used in the constituting liquid crystal composition and that an increase of the chiral dopant adversely affects the display characteristics such as a response.
Further, because the current increases with temperature change-induced change in mobility of the ionic substance contained in liquid crystals, the effective voltage applied to the liquid crystals is consumed by ions, resulting in a reduction of the contrast Therefore, current control is an important factor for contrast improvement.
To improve response characteristics in supertwisted nematic (STN) LCDs, an active addressing drive system has recently been proposed (Proc. 12th International Display Research conference, p. 503 (1992) It has been required for PDAs to have satisfactory display characteristics in a broader temperature range. Liquid crystal materials to be used in these applications are required to have a low viscosity and capability of being driven at a low driving voltage and yet keeping a constant voltage over a broad temperature range. Improvements in these respects are proposed in JP-A-3-38572 , U.S. Pat. No. 5,030,382, JP-A-12-336364, JP-A-10-330754, and JP-A-11-106752. The proposed techniques, however, cannot be seen as satisfactory in terms of switching time and contrast at a desired threshold voltage or in a desired time-sharing system. Proposals on novel liquid crystal displays are still awaited.
An object of the present invention is to provide a liquid crystal display which alleviates the disadvantages associated with the state-of-the-art liquid crystal displays, i.e., which exhibits reduced temperature dependence of driving voltage, a high contrast (steepness) and a short switching time.
Specifically, the object is to achieve improvement on temperature dependence of threshold voltage without impairing various characteristics of liquid crystals when driven at a desired threshold voltage or a desired duty (time-sharing drive), thereby securing a high contract stably in a broader temperature range and reducing a switching time at room temperature and low temperature ranges.
Another object of the invention is to provide an LCD (STN-LCD) having excellent characteristics, such as an increased display capacity and capability of displaying a colored motion picture, that are requested of, for example, cellular phones of next generation, which is driven at, e.g., {fraction (1/32)} to {fraction (1/480)} duty, preferably {fraction (1/64)} to {fraction (1/240)} duty.
Other objects and effects of the present invention will become apparent from the following description.
As a result of extensive researches into liquid crystal displays comprising various liquid crystal compositions, the present inventors found the following liquid crystal display:
A supertwisted nematic liquid crystal display having a twist angle of 180 to 270xc2x0 C. and comprising:
a pair of substrates each having an orientation-controlling layer and a transparent electrode;
a liquid crystal layer comprising a liquid crystal composition, which is sandwiched by said substrates; and
at least one polarizing sheet provided on at least one of said substrates,
wherein said liquid crystal composition comprises:
at least one compound represented by formula (I): 
xe2x80x83wherein R1 represents an alkyl group having 1 to 16 carbon atoms, an alkoxy group having 1 to 16 carbon atoms, an alkenyl group having 2 to 16 carbon atoms, an alkoxyalkyl group having 2 to 16 carbon atoms, or an alkenyloxy group having 3 to 16 carbon atoms; and
at least one compound represented by formula (II): 
xe2x80x83wherein R2 and R3 each independently represent a fluorine-substituted or unsubstituted alkyl group having 1 to 16 carbon atoms, a fluorine-substituted or unsubstituted alkoxy group having 1 to 16 carbon atoms, or a fluorine-substituted or unsubstituted alkenyl group having 2 to 16 carbon atoms, and
wherein said liquid crystal composition has a nematic-isotropic phase transition temperature of 75 to 150xc2x0 C. and a refractive index anisotropy (xcex94n) of 0.07 to 0.25.
Liquid crystal displays using the compounds of formula (I), while effective in reducing a threshold voltage, are accompanied with a viscosity increase. On the other hand, liquid crystal displays using the compounds of formula (II) are low-viscosity compounds effective in shortening a switching time but have small polarity which increases the driving voltage. Under these circumstances, the inventors reached findings that a liquid crystal display using at least one of the compounds of formula (I) and at least one of the compounds of formula (II) in combination compensates for the respective drawbacks with each other and also bring about a surprising improvement on temperature dependence of threshold voltage, especially on switching time in low temperature.